This invention relates to the ultra-purification of boron nitride to increase its useful life in alkali metal/fused salt electrochemical cells and particularly in the less reactive alloyed-lithium cells.
Boron nitride (BN) has use as an insulator and/or separator material in alkali metal-fused salt electrochemical cells. Battles et al U.S. Pat. No. 3,915,742 discloses the need to free BN from any boric oxide therein to prevent degradation of the BN separator/insulator by the alkali metal in such cells. Others have proposed to remove the B.sub.2 O.sub.3 by heating the BN to drive off the B.sub.2 O.sub.3, and Battles et al performs the heating in a dry, flowing nitrogen atmosphere to suppress BN decomposition at the B.sub.2 O.sub.3 -freeing temperature and has apparently achieved boric oxide contents of as low as 0.3 weight percent and as much as 1390 hours of useful life as a separator in a Li-Al/LiCl-KCl/FeS.sub.2 cell. The nitrogen pressure, however, retards more complete removal of the B.sub.2 O.sub.3 and, if any O.sub.2 or moisture at all is present, allows the risk of forming B.sub.2 O.sub.3 from the BN rather than removing it.
It is an object of the present invention to provide a process for the ultra-purification of BN to not only substantially increase its useful life in lithium-alloy fused salt cells, but also to render it more long-lived in pure alkali metal/fused salt cells.